The No.4-seeded Stosur came out of the blocks far stronger, storming through the first set in just 28 minutes, 6-1, and breaking in the first game of the second set. She was even up 30-15 in her next service game, seemingly on the road to a straight set win, when Pavlyuchenkova found her groove.
The No.6-seeded Russian retaliated with six games in a row, running away with a 6-1 second set, and the two went to a decider - but Stosur had just one more push left in her, getting the only break of the set in the fifth game, then holding serve the rest of the way until she sealed a 61 16 63 victory.
For the second straight day at the Armeets Arena, Stosur clinched the victory with a big ace.
"It was a tough match," Stosur said in her on-court interview. "I got off to a good start, and she was missing a little bit in the first set, but then things totally changed around in the second set. The third set was the time where we both played our best. But I'm happy I was able to close it out in the end."
Stosur won 20 of 25 points on her serve in the third set. "I served pretty well tonight," the former World No.4 said. "It certainly got me out of trouble in a lot of those service games, which was good."
Stosur has now won 12 of her last 14 matches, a stretch that includes winning the title in Osaka, reaching the final in Moscow (falling to Simona Halep) and going 3-1 here (she lost one of her round robin matches to Ana Ivanovic). And now, perhaps fittingly, Stosur plays Halep again for this title.
"I've got to keep serving well, and I also have to try and return better," Stosur said. "Simona barely misses anything on the ground, so you've really got to work hard for every point. So it's going to be a tough match - I have to try and be aggressive against her, but also not make many unforced errors."
Halep - who won her semifinal match earlier in the day - was asked about playing against Stosur.
"I'm more confident against her right now because I won our last match in Moscow, but it will be a tough match for sure," Halep said. "I'm sure she wants to beat me this time, so I have to be ready."
Halep is 5-3 in WTA finals (5-0 this year) while Stosur is 5-13 in WTA finals (2-1 this year).
Stosur leads Halep in their head-to-head series, 3-2, though Halep is 2-0 against Stosur this year.